Magnetic recording media, having on a non-magnetic support a magnetic layer comprising a dispersion in a binding agent (binder) of a ferromagnetic powder consisting of needle shaped crystals of .gamma.-Fe.sub.2 O.sub.3, Co-containing magnetic iron oxide, CrO.sub.2 etc., have been used in general as magnetic recording media (also referred to below as magnetic tapes) in audio, video and computer applications.
Magnetic recording media of this type can be recorded at higher densities if the surface of the magnetic layer is made smoother when forming the magnetic layer and it is known that this improvement enhances the electromagnetic conversion characteristics of the magnetic recording medium.
However, if the surface of the magnetic layer is made smoother, then the coefficient of friction for the contact between the magnetic layer and the apparatus while a video tape is being run is increased and consequently the magnetic layer of the magnetic recording medium is liable to be damaged in use over a short period of time. Furthermore, the magnetic layer may tend to peel away.
With video tapes in particular, the recording medium is typically run at a high speed while in contact with the video head. Due to the high friction thus created, ferromagnetic dust is liable to be shed from the magnetic layer and this results in blocking of the video head. Hence, improvement of the running durability of the magnetic layers of video tapes is clearly desirable.
In the past, methods in which polishing materials (hard particles) such as corundum, silicon carbide, chromium oxide etc. are added to the magnetic layer have been proposed as a means of improving the running durability of magnetic layers. But when polishing materials are added to a magnetic layer with a view to improving running durability, the materials must be added in large amounts or it is difficult to achieve any effect. However, magnetic layers to which large amounts of polishing materials have been added result in pronounced wearing of the magnetic heads etc. Furthermore, since adding large amounts of such materials also adversely affects the smoothing of the magnetic layer and the electromagnetic conversion characteristics, this method is undesirable.
Fatty acids, or esters of fatty acids and aliphatic alcohols, have been added to magnetic layers as lubricants in order to reduce to coefficient of friction. However, adding large amounts of lubricant to increase the lubricating effect reduces the S/N ratio, and further this practice has an adverse effect on permalloy heads while it has a good effect on stainless guides. For this reason, it is difficult to achieve a balanced running performance with such additives.
Carbon black has long been included in the magnetic layers of magnetic recording media of this type as described in U.S. Patents 4,614,685 and 4,539,257, principally with a view to preventing the build-up of static. Inclusion of carbon black is also known to have an effect on the running durability. However, magnetic layers which contain carbon black have poor surface smoothness and, in the case of video tapes in particular where high density recording is essential, there is a marked tendency for the electromagnetic conversion characteristics to deteriorate as the amount of carbon black added is increased. Furthermore, in the case of audio tapes, magnetic layers which contain large amounts of carbon black have a lower degree of packing of magnetic particles within the magnetic layer, and therefore there is a deterioration in the electromagnetic conversion characteristics as described earlier. However, magnetic layers which do not contain carbon black have no anti-static effect and so their durability is very poor.
Hence, sufficiently good running durability cannot be obtained using the conventional lubricants, polishing materials and carbon black.